Strong effects of soil microbes on early-life defense alter phenotypic correlations in plant leaves

Microbes have wide-ranging effects on host phenotype. However, whether these effects extend to the relationships among host traits remains unknown. We tested whether microbes affect phenotypic correlations among early life traits in the weedy legume Medicago lupulina. In a field common garden, we inoculated plants with two types of microbes: nitrogen-fixing bacteria and rhizosphere microbial communities. We found that microbes modify phenotypic integration (phenotypic correlations) in leaves, primarily due to effects on independent effects on leaflet area and trichome density, a defense trait. While microbial effects on leaflet size were associated with overall plant growth, variation in trichome density was decoupled from growth and not predicted by investment in mutualism. Our results highlight the unique insights that can come from a multivariate approach to organismal plasticity, and raise the intriguing possibility that microbes may have an outsized impact on defense traits and their integration with organismal function.